Salicylic acid carboxyl glucosyltransferase UGT87E7 regulates disease resistance in Camellia sinensis

Exploring the Formation of Chemical Markers in Chaste Honey by Comparative Metabolomics: From Nectar to Mature Honey

Identification of chemical markers is important to ensure the authenticity of monofloral honey; however, the formation of chemical markers in honey has received little attention. Herein, using comparative metabolomics, we first identified chemical markers in chaste honey and then explored their formation and accumulation from nectar to mature honey. We identified agnuside and p-hydroxybenzoic acid glucosides as chemical markers for chaste honey. Besides, we developed an UHPLC-MS/MS method for quantifying these markers and found that their levels varied significantly across sample sources. We compared the presence of these compounds in chaste nectar and mature honey. The outcomes underscore that these characteristic compounds are not simply delivered from nectar to mature honey, and activities of honeybees (collecting and processing) play a pivotal role in their formation and accumulation. These observations shed light on how mature honey can form its unique qualities with a rich assortment of natural bioactive compounds, potentially supporting health benefits.

Bulked Segregant RNA-Seq Reveals Different Gene Expression Patterns and Mutant Genes Associated with the Zigzag Pattern of Tea Plants (Camellia sinensis)

The unique zigzag-patterned tea plant is a rare germplasm resource. However, the molecular mechanism behind the formation of zigzag stems remains unclear. To address this, a BC1 genetic population of tea plants with zigzag stems was studied using histological observation and bulked segregant RNA-seq. The analysis revealed 1494 differentially expressed genes (DEGs) between the upright and zigzag stem groups. These DEGs may regulate the transduction and biosynthesis of plant hormones, and the effects on the phenylpropane biosynthesis pathways may cause the accumulation of lignin. Tissue sections further supported this finding, showing differences in cell wall thickness between upright and curved stems, potentially due to lignin accumulation. Additionally, 262 single-nucleotide polymorphisms (SNPs) across 38 genes were identified as key SNPs, and 5 genes related to zigzag stems were identified through homologous gene function annotation. Mutations in these genes may impact auxin distribution and content, resulting in the asymmetric development of vascular bundles in curved stems. In summary, we identified the key genes associated with the tortuous phenotype by using BSR-seq on a BC1 population to minimize genetic background noise.

A tau class glutathione S-transferase in tea plant, CsGSTU45, facilitates tea plant susceptibility to Colletotrichum camelliae infection mediated by jasmonate signaling pathway

Tea plant [Camellia sinensis (L.) O. Kuntze], as one of the most important commercial crops, frequently suffers from anthracnose caused by Colletotrichum camelliae. The plant-specific tau (U) class of glutathione S-transferases (GSTU) participates in ROS homeostasis. Here, we identified a plant-specific GST tau class gene from tea plant, CsGSTU45, which is induced by various stresses, including C. camelliae infection, by analyzing multiple transcriptomes. CsGSTU45 plays a negative role in disease resistance against C. camelliae by accumulating H2 O2 . JA negatively regulates the resistance of tea plants against C. camelliae, which depends on CsGSTU45. CsMYC2.2, which is the key regulator in the JA signaling pathway, directly binds to and activates the promoter of CsGSTU45. Furthermore, silencing CsMYC2.2 increased disease resistance associated with reduced transcript and protein levels of CsGSTU45, and decreased contents of H2 O2 . Therefore, CsMYC2.2 suppresses disease resistance against C. camelliae by binding to the promoter of the CsGSTU45 gene and activating CsGSTU45. CsJAZ1 interacts with CsMYC2.2. Silencing CsJAZ1 attenuates disease resistance, upregulates the expression of CsMYC2.2 elevates the level of the CsGSTU45 protein, and promotes the accumulation of H2 O2 . As a result, CsJAZ1 interacts with CsMYC2.2 and acts as its repressor to suppress the level of CsGSTU45 protein, eventually enhancing disease resistance in tea plants. Taken together, the results show that the JA signaling pathway mediated by CsJAZ1-CsMYC2.2 modulates tea plant susceptibility to C. camelliae by regulating CsGSTU45 to accumulate H2 O2 .

CsPAT1, a GRAS transcription factor, promotes lignin accumulation by antagonistic interacting with CsWRKY13 in tea plants

Lignin is an important component of plant cell walls and plays crucial roles in the essential agronomic traits of tea quality and tenderness. However, the molecular mechanisms underlying the regulation of lignin biosynthesis in tea plants remain unclear. CsWRKY13 acts as a negative regulator of lignin biosynthesis in tea plants. In this study, we identified a GRAS transcription factor, phytochrome A signal transduction 1 (CsPAT1), that interacts with CsWRKY13. Silencing CsPAT1 expression in tea plants and heterologous overexpression in Arabidopsis demonstrated that CsPAT1 positively regulates lignin accumulation. Further investigation revealed that CsWRKY13 directly binds to the promoters of CsPAL and CsC4H and suppresses transcription of CsPAL and CsC4H. CsPAT1 indirectly affects the promoter activities of CsPAL and CsC4H by interacting with CsWRKY13, thereby facilitating lignin biosynthesis in tea plants. Compared with the expression of CsWRKY13 alone, the co-expression of CsPAT1 and CsWRKY13 in Oryza sativa significantly increased lignin biosynthesis. Conversely, compared with the expression of CsPAT1 alone, the co-expression of CsPAT1 and CsWRKY13 in O. sativa significantly reduced lignin accumulation. These results demonstrated the antagonistic regulation of the lignin biosynthesis pathway by CsPAT1 and CsWRKY13. These findings improve our understanding of lignin biosynthesis mechanisms in tea plants and provide insights into the role of the GRAS transcription factor family in lignin accumulation.

Mining of disease-resistance genes in Crocus sativus based on transcriptome sequencing

Introduction: Crocus sativus L. has an important medicinal and economic value in traditional perennial Chinese medicine. However, due to its unique growth characteristics, during cultivation it is highly susceptible to disease. The absence of effective resistance genes restricts us to breed new resistant varieties of C. sativus. Methods: In present study, comprehensive transcriptome sequencing was introduced to explore the disease resistance of the candidate gene in healthy and corm rot-infected C. sativus. Results and discussion: Totally, 43.72 Gb of clean data was obtained from the assembly to generate 65,337 unigenes. By comparing the gene expression levels, 7,575 differentially expressed genes (DEGs) were primarily screened. A majority of the DEGs were completely in charge of defense and metabolism, and 152 of them were annotated as pathogen recognition genes (PRGs) based on the PGRdb dataset. The expression of some transcription factors including NAC, MYB, and WRKY members, changed significantly based on the dataset of transcriptome sequencing. Therefore, this study provides us some valuable information for exploring candidate genes involved in the disease resistance in C. sativus.

2,4-Dihydroxybenzoic Acid, a Novel SA Derivative, Controls Plant Immunity via UGT95B17-Mediated Glucosylation: A Case Study in Camellia Sinensis

The plant hormone salicylic acid (SA) plays critical roles in plant innate immunity. Several SA derivatives and associated modification are identified, whereas the range and modes of action of SA-related metabolites remain elusive. Here, the study discovered 2,4-dihydroxybenzoic acid (2,4-DHBA) and its glycosylated form as native SA derivatives in plants whose accumulation is largely induced by SA application and Ps. camelliae-sinensis (Pcs) infection. CsSH1, a 4/5-hydroxylase, catalyzes the hydroxylation of SA to 2,4-DHBA, and UDP-glucosyltransferase UGT95B17 catalyzes the formation of 2,4-DHBA glucoside. Down-regulation reduced the accumulation of 2,4-DHBA glucosides and enhanced the sensitivity of tea plants to Pcs. Conversely, overexpression of UGT95B17 increased plant disease resistance. The exogenous application of 2,4-DHBA and 2,5-DHBA, as well as the accumulation of DHBA and plant resistance comparison, indicate that 2,4-DHBA functions as a potentially bioactive molecule and is stored mainly as a glucose conjugate in tea plants, differs from the mechanism described in Arabidopsis. When 2,4-DHBA is applied exogenously, UGT95B17-silenced tea plants accumulated more 2,4-DHBA than SA and showed induced resistance to Pcs infection. These results indicate that 2,4-DHBA glucosylation positively regulates disease resistance and highlight the role of 2,4-DHBA as potentially bioactive molecule in the establishment of basal resistance in tea plants.

CsMYB67 participates in the flavonoid biosynthesis of summer tea leaves

Flavonoids are important compounds in tea leaves imparting bitter and astringent taste, which also play key roles in tea plants responding to environmental stress. Our previous study showed that the expression level of CsMYB67 was positively correlated with the accumulation of flavonoids in tea leaves as exposed to sunlight. Here, we newly reported the function of CsMYB67 in regulating flavonoid biosynthesis in tea leaves. CsMYB67 was localized in the nucleus and responded to temperature. The results of transient expression assays showed the co-transformation of CsMYB67 and CsTTG1 promoted the transcription of CsANS promoter in the tobacco system. CsTTG1 was bound to the promoter of CsANS based on the results of yeast one-hybrid (Y1H) and transient expression assays, while CsMYB67 enhanced the transcription of CsANS through protein interaction with CsTTG1 according to the results of yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC). Thus, CsMYB67-CsTTG1 module enhanced the anthocyanin biosynthesis through up-regulating the transcription of CsANS. Besides, CsMYB67 also enhanced the transcription of CsFLS and CsUFGT through forming transcription factor complexes. The function of CsMYB67 on flavonoid biosynthesis in tea leaves was validated by gene suppression assay. As CsMYB67 was suppressed, the transcriptional level of CsFLS was greatly reduced, leading to a significant increase in the contents of total catechins and total anthocyanidins. Hence, CsMYB67 plays an important role in regulating the downstream pathway of flavonoid biosynthesis in summer tea leaves.

Glycosylation of Secondary Metabolites: A Multifunctional UDP-Glycosyltransferase, CsUGT74Y1, Promotes the Growth of Plants

Camellia sinensis contains numerous glycosylated secondary metabolites that provide various benefits to plants and humans. However, the genes that catalyze the glycosylation of multitype metabolites in tea plants remain unclear. Here, 180 uridine diphosphate-dependent glycosyltransferases that may be involved in the biosynthesis of glycosylated secondary metabolites were identified from the National Center for Biotechnology Information public databases. Subsequently, CsUGT74Y1 was screened through phylogenetic analysis and gene expression profiling. Compositional and induced expression analyses revealed that CsUGT74Y1 was highly expressed in tea tender shoots and was induced under biotic and abiotic stress conditions. In vitro enzymatic assays revealed that rCsUGT74Y1 encoded a multifunctional UGT that catalyzed the glycosylation of flavonoids, phenolic acids, lignins, and auxins. Furthermore, CsUGT74Y1-overexpressing Arabidopsis thaliana exhibited enhanced growth and accumulation of flavonol and auxin glucosides. Our findings provide insights into identifying specific UGTs and demonstrate that CsUGT74Y1 is a multifunctional UGT that promotes plant development.

Pour some sugar on me: The diverse functions of phenylpropanoid glycosylation

The phenylpropanoid metabolism is the source of a vast array of specialized metabolites that play diverse functions in plant growth and development and contribute to all aspects of plant interactions with their surrounding environment. These compounds protect plants from damaging ultraviolet radiation and reactive oxygen species, provide mechanical support for the plants to stand upright, and mediate plant-plant and plant-microorganism communications. The enormous metabolic diversity of phenylpropanoids is further expanded by chemical modifications known as "decorative reactions", including hydroxylation, methylation, glycosylation, and acylation. Among these modifications, glycosylation is the major driving force of phenylpropanoid structural diversification, also contributing to the expansion of their properties. Phenylpropanoid glycosylation is catalyzed by regioselective uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs), whereas glycosyl hydrolases known as β-glucosidases are the major players in deglycosylation. In this article, we review how the glycosylation process affects key physicochemical properties of phenylpropanoids, such as molecular stability and solubility, as well as metabolite compartmentalization/storage and biological activity/toxicity. We also summarize the recent knowledge on the functional implications of glycosylation of different classes of phenylpropanoid compounds. A balance of glycosylation/deglycosylation might represent an essential molecular mechanism to regulate phenylpropanoid homeostasis, allowing plants to dynamically respond to diverse environmental signals.

A geraniol synthase regulates plant defense via alternative splicing in tea plants

Geraniol is an important contributor to the pleasant floral scent of tea products and one of the most abundant aroma compounds in tea plants; however, its biosynthesis and physiological function in response to stress in tea plants remain unclear. The proteins encoded by the full-length terpene synthase (CsTPS1) and its alternative splicing isoform (CsTPS1-AS) could catalyze the formation of geraniol when GPP was used as a substrate in vitro, whereas the expression of CsTPS1-AS was only significantly induced by Colletotrichum gloeosporioides and Neopestalotiopsis sp. infection. Silencing of CsTPS1 and CsTPS1-AS resulted in a significant decrease of geraniol content in tea plants. The geraniol content and disease resistance of tea plants were compared when CsTPS1 and CsTPS1-AS were silenced. Down-regulation of the expression of CsTPS1-AS reduced the accumulation of geraniol, and the silenced tea plants exhibited greater susceptibility to pathogen infection than control plants. However, there was no significant difference observed in the geraniol content and pathogen resistance between CsTPS1-silenced plants and control plants in the tea plants infected with two pathogens. Further analysis showed that silencing of CsTPS1-AS led to a decrease in the expression of the defense-related genes PR1 and PR2 and SA pathway-related genes in tea plants, which increased the susceptibility of tea plants to pathogens infections. Both in vitro and in vivo results indicated that CsTPS1 is involved in the regulation of geraniol formation and plant defense via alternative splicing in tea plants. The results of this study provide new insights into geraniol biosynthesis and highlight the role of monoterpene synthases in modulating plant disease resistance via alternative splicing.

A constitutive serine protease inhibitor suppresses herbivore performance in tea (Camellia sinensis)

Protease inhibitors promote herbivore resistance in diverse plant species. Although many inducible protease inhibitors have been identified, there are limited reports available on the biological relevance and molecular basis of constitutive protease inhibitors in herbivore resistance. Here, we identified a serine protease inhibitor, CsSERPIN1, from the tea plant (Camellia sinensis). Expression of CsSERPIN1 was not strongly affected by the assessed biotic and abiotic stresses. In vitro and in vivo experiments showed that CsSERPIN1 strongly inhibited the activities of digestive protease activities of trypsin and chymotrypsin. Transient or heterologous expression of CsSERPIN1 significantly reduced herbivory by two destructive herbivores, the tea geometrid and fall armyworm, in tea and Arabidopsis plants, respectively. The expression of CsSERPIN1 in Arabidopsis did not negatively influence the growth of the plants under the measured parameters. Our findings suggest that CsSERPIN1 can inactivate gut digestive proteases and suppress the growth and development of herbivores, making it a promising candidate for pest prevention in agriculture.

(Z)-3-Hexenol integrates drought and cold stress signaling by activating abscisic acid glucosylation in tea plants

Cold and drought stresses severely limit crop production and can occur simultaneously. Although some transcription factors and hormones have been characterized in plants subjected each stress, the role of metabolites, especially volatiles, in response to cold and drought stress exposure is rarely studied due to lack of suitable models. Here, we established a model for studying the role of volatiles in tea (Camellia sinensis) plants experiencing cold and drought stresses simultaneously. Using this model, we showed that volatiles induced by cold stress promote drought tolerance in tea plants by mediating reactive oxygen species and stomatal conductance. Needle trap microextraction combined with GC-MS identified the volatiles involved in the crosstalk and showed that cold-induced (Z)-3-hexenol improved the drought tolerance of tea plants. In addition, silencing C. sinensis alcohol dehydrogenase 2 (CsADH2) led to reduced (Z)-3-hexenol production and significantly reduced drought tolerance in response to simultaneous cold and drought stress. Transcriptome and metabolite analyses, together with plant hormone comparison and abscisic acid (ABA) biosynthesis pathway inhibition experiments, further confirmed the roles of ABA in (Z)-3-hexenol-induced drought tolerance of tea plants. (Z)-3-Hexenol application and gene silencing results supported the hypothesis that (Z)-3-hexenol plays a role in the integration of cold and drought tolerance by stimulating the dual-function glucosyltransferase UGT85A53, thereby altering ABA homeostasis in tea plants. Overall, we present a model for studying the roles of metabolites in plants under multiple stresses and reveal the roles of volatiles in integrating cold and drought stresses in plants.

Proteomics Identified UDP-Glycosyltransferase Family Members as Pro-Viral Factors for Turnip Mosaic Virus Infection in Nicotiana benthamiana

Viruses encounter numerous host factors that facilitate or suppress viral infection. Although some host factors manipulated by viruses were uncovered, we have limited knowledge of the pathways hijacked to promote viral replication and activate host defense responses. Turnip mosaic virus (TuMV) is one of the most prevalent viral pathogens in many regions of the world. Here, we employed an isobaric tag for relative and absolute quantitation (iTRAQ)-based proteomics approach to characterize cellular protein changes in the early stages of infection of Nicotiana benthamiana by wild type and replication-defective TuMV. A total of 225 differentially accumulated proteins (DAPs) were identified (182 increased and 43 decreased). Bioinformatics analysis showed that a few biological pathways were associated with TuMV infection. Four upregulated DAPs belonging to uridine diphosphate-glycosyltransferase (UGT) family members were validated by their mRNA expression profiles and their effects on TuMV infection. NbUGT91C1 or NbUGT74F1 knockdown impaired TuMV replication and increased reactive oxygen species production, whereas overexpression of either promoted TuMV replication. Overall, this comparative proteomics analysis delineates the cellular protein changes during early TuMV infection and provides new insights into the role of UGTs in the context of plant viral infection.

Establishment of an efficient transformation system and its application in regulatory mechanism analysis of biological macromolecules in tea plants

Tea (Camellia sinensis), one of the most important beverage crops originated from China and is now cultivated worldwide, provides numerous secondary metabolites that account for its health benefits and rich flavor. However, the lack of an efficient and reliable genetic transformation system has seriously hindered the gene function investigation and precise breeding of C. sinensis. In this study, we established a highly efficient, labor-saving, and cost-effective Agrobacterium rhizogenes-mediated hairy roots genetic transformation system for C. sinensis, which can be used for gene overexpression and genome editing. The established transformation system was simple to operate, bypassing tissue culture and antibiotic screening, and only took two months to complete. We used this system to conduct function analysis of transcription factor CsMYB73 and found that CsMYB73 negatively regulates L-theanine synthesis in tea plant. Additionally, callus formation was successfully induced using transgenic roots, and the transgenic callus exhibited normal chlorophyll production, enabling the study of the corresponding biological functions. Furthermore, this genetic transformation system was effective for multiple C. sinensis varieties and other woody plant species. By overcoming technical obstacles such as low efficiency, long experimental periods, and high costs, this genetic transformation will be a valuable tool for routine gene investigation and precise breeding in tea plants.

Genome-wide identification of UDP-glycosyltransferases in the tea plant (Camellia sinensis) and their biochemical and physiological functions

Tea (Camellia sinensis) has been an immensely important commercially grown crop for decades. This is due to the presence of essential nutrients and plant secondary metabolites that exhibit beneficial health effects. UDP-glycosyltransferases (UGTs) play an important role in the diversity of such secondary metabolites by catalysing the transfer of an activated sugar donor to acceptor molecules, and thereby creating a huge variety of glycoconjugates. Only in recent years, thanks to the sequencing of the tea plant genome, have there been increased efforts to characterise the UGTs in C. sinensis to gain an understanding of their physiological role and biotechnological potential. Based on the conserved plant secondary product glycosyltransferase (PSPG) motif and the catalytically active histidine in the active site, UGTs of family 1 in C. sinensis are identified here, and shown to cluster into 21 groups in a phylogenetic tree. Building on this, our current understanding of recently characterised C. sinensis UGTs (CsUGTs) is highlighted and a discussion on future perspectives made.

The CsHSFA-CsJAZ6 module-mediated high temperature regulates flavonoid metabolism in Camellia sinensis

High temperatures (HTs) seriously affect the yield and quality of tea. Catechins, derived from the flavonoid pathway, are characteristic compounds that contribute to the flavour of tea leaves. In this study, we first showed that the flavonoid content of tea leaves was significantly reduced under HT conditions via metabolic profiles; and then demonstrated that two transcription factors, CsHSFA1b and CsHSFA2 were activated by HT and negatively regulate flavonoid biosynthesis during HT treatment. Jasmonate (JA), a defensive hormone, plays a key role in plant adaption to environmental stress. However, little has been reported on its involvement in HT response in tea. Herein, we demonstrated that CsHSFA1b and CsHSFA2 activate CsJAZ6 expression through directly binding to heat shock elements in its promoter, and thereby repress the JA pathway. Most secondary metabolites are regulated by JA, including catechin in tea. Our study reported that CsJAZ6 directly interacts with CsEGL3 and CsTTG1 and thereby reduces catechin accumulation. From this, we proposed a CsHSFA-CsJAZ6-mediated HT regulation model of catechin biosynthesis. We also determined that negative regulation of the JA pathway by CsHSFAs and its homologues is conserved in Arabidopsis. These findings broaden the applicability of the regulation of JAZ by HSF transcription factors and further suggest the JA pathway as a valuable candidate for HT-resistant breeding and cultivation.

Molecular regulation of immunity in tea plants

Tea, which is mainly produced using the young leaves and buds of tea plants (Camellia sinensis (L.) O. Kuntze), is one of the most common non-alcoholic beverages consumed in the world. The standard of tea mostly depends on the variety and quality of tea plants, which generally grow in subtropical areas, where the warm and humid conditions are also conducive to the occurrence of diseases. In fighting against pathogens, plants rely on their sophisticated innate immune systems which has been extensively studied in model plants. Many components involved in pathogen associated molecular patterns (PAMPs) triggered immunity (PTI) and effector triggered immunity (ETI) have been found. Nevertheless, the molecular regulating network against pathogens (e.g., Pseudopestalotiopsis sp., Colletotrichum sp. and Exobasidium vexans) causing widespread disease (such as grey blight disease, anthracnose, and blister blight) in tea plants is still unclear. With the recent release of the genome data of tea plants, numerous genes involved in tea plant immunity have been identified, and the molecular mechanisms behind tea plant immunity is being studied. Therefore, the recent achievements in identifying and cloning functional genes/gene families, in finding crucial components of tea immunity signaling pathways, and in understanding the role of secondary metabolites have been summarized and the opportunities and challenges in the future studies of tea immunity are highlighted in this review.

Salicylic acid interacts with other plant growth regulators and signal molecules in response to stressful environments in plants

Salicylic acid (SA) is one of the potential plant growth regulators (PGRs) that regulate plant growth and development by triggering many physiological and metabolic processes. It is also known to be a crucial component of plant defense mechanisms against environmental stimuli. In stressed plants, it is documented that it can effectively modulate a myriad of metabolic processes including strengthening of oxidative defense system by directly or indirectly limiting the buildup of reactive nitrogen and oxygen radicals. Although it is well recognized that it performs a crucial role in plant tolerance to various stresses, it is not fully elucidated that whether low or high concentrations of this PGR is effective to achieve optimal growth of plants under stressful environments. It is also not fully understood that to what extent and in what manner it cross-talks with other potential growth regulators and signalling molecules within the plant body. Thus, this critical review discusses how far SA mediates crosstalk with other key PGRs and molecular components of signalling pathways mechanisms, particularly in plants exposed to environmental cues. Moreover, the function of SA exogenously applied in regulation of growth and development as well as reinforcement of oxidative defense system of plants under abiotic stresses is explicitly elucidated.

Understanding the Origin and Evolution of Tea (Camellia sinensis [L.]): Genomic Advances in Tea

Tea, which is processed by the tender shoots or leaves of tea plant (Camellia sinensis), is one of the most popular nonalcoholic beverages in the world and has numerous health benefits for humans. Along with new progress in biotechnologies, the refined chromosome-scale reference tea genomes have been achieved, which facilitates great promise for the understanding of fundamental genomic architecture and evolution of the tea plants. Here, we summarize recent achievements in genome sequencing in tea plants and review the new progress in origin and evolution of tea plants by population sequencing analysis. Understanding the genomic characterization of tea plants is import to improve tea quality and accelerate breeding in tea plants.

Transcriptional Analysis of Tea Plants (Camellia sinensis) in Response to Salicylic Acid Treatment

Salicylic acid (SA) is an important plant hormone and signal required for establishing resistance to diverse pathogens and plant diseases. The abundant polyphenols in tea plants also defend plants from biotic and abiotic stresses. However, whether exogenous SA would increase the resistance of tea plants to adversity and the relationship between SA and polyphenols are still poorly understood. Here, we carried out SA treatment on tea seedlings and performed transcriptome sequencing. SA treatment inhibited the phenylpropanoid and flavonoid metabolic pathways but promoted the lignin metabolic pathways. The increased accumulation of lignin in tea leaves after treating with SA indicated that lignin might coordinate SA, enhance, and improve plant defense and disease resistance. Simultaneously, an SA-inducible flavonoid glucosyltransferase (CsUGT0554) specifically involved in 7-OH site glycosylation was characterized in vitro. These results provided valuable information about the effects of SA on tea seedlings and the molecular basis for SA-mediated immune responses.

Haem Oxygenase 1 is a potential target for creating etiolated/albino tea plants (Camellia sinensis) with high theanine accumulation

Theanine content is highly correlated with sensory quality and health benefits of tea infusion. The tender shoots of etiolated and albino tea plants contain higher theanine than the normal green tea plants and are valuable materials for high quality green tea processing. However, why these etiolated or albino tea plants can highly accumulate theanine is largely unknown. In this study, we observed an Arabidopsis etiolated mutant hy1-100 (mutation in Haem Oxygenase 1, HO1) that accumulated higher levels of glutamine (an analog of theanine). We therefore identified CsHO1 in tea plants and found CsHO1 is conserved in amino acid sequences and subcellular localization with its homologs in other plants. Importantly, CsHO1 expression in the new shoots was much lower in an etiolated tea plants 'Huangkui' and an albino tea plant 'Huangshan Baicha' than that in normal green tea plants. The expression levels of CsHO1 were negatively correlated with theanine contents in these green, etiolated and albino shoots. Moreover, CsHO1 expression levels in various organs and different time points were also negatively correlated with theanine accumulation. The hy1-100 was hypersensitive to high levels of theanine and accumulated more theanine under theanine feeding, and these phenotypes were rescued by the expression of CsHO1 in this mutant. Transient knockdown CsHO1 expression in the new shoots of tea plant using antisense oligonucleotides (asODN) increased theanine accumulation. Collectively, these results demonstrated CsHO1 negatively regulates theanine accumulation in tea plants, and that low expression CsHO1 likely contributes to the theanine accumulation in etiolated/albino tea plants.

Evolutionary Landscape of Tea Circular RNAs and Its Contribution to Chilling Tolerance of Tea Plant

Chilling stress threatens the yield and distribution pattern of global crops, including the tea plant (Camellia sinensis), one of the most important cash crops around the world. Circular RNA (circRNA) plays roles in regulating plant growth and biotic/abiotic stress responses. Understanding the evolutionary characteristics of circRNA and its feedbacks to chilling stress in the tea plant will help to elucidate the vital roles of circRNAs. In the current report, we systematically identified 2702 high-confidence circRNAs under chilling stress in the tea plant, and interestingly found that the generation of tea plant circRNAs was associated with the length of their flanking introns. Repetitive sequences annotation and DNA methylation analysis revealed that the longer flanking introns of circRNAs present more repetitive sequences and higher methylation levels, which suggested that repeat-elements-mediated DNA methylation might promote the circRNAs biogenesis in the tea plant. We further detected 250 differentially expressed circRNAs under chilling stress, which were functionally enriched in GO terms related to cold/stress responses. Constructing a circRNA-miRNA-mRNA interaction network discovered 139 differentially expressed circRNAs harboring potential miRNA binding sites, which further identified 14 circRNAs that might contribute to tea plant chilling responses. We further characterized a key circRNA, CSS-circFAB1, which was significantly induced under chilling stress. FISH and silencing experiments revealed that CSS-circFAB1 was potentially involved in chilling tolerance of the tea plant. Our study emphasizes the importance of circRNA and its preliminary role against low-temperature stress, providing new insights for tea plant cold tolerance breeding.

Comparative Physicochemical and Biochemical Characterization of Small-Molecule Glucosides

Glycosylation of small molecules can significantly improve their physicochemical and biological properties. Only recently, decisive improvements in the biotechnological production of small-molecule glucosides (SMGs) have resulted in a large number of these compounds now being commercially available. In this study, we have analyzed a number of physical, chemical, and biological parameters of 31 SMGs, including solubility, stability, melting and pyrolysis points, partition coefficient log P, minimum inhibitory concentration against Escherichia coli (MIC), and enzymatic degradability. The properties such as water solubility, pH stability, and MICs of the glycosides were strongly dependent on the structures of the respective aglycones, which is why the SMG clustered according to their aglycones in most cases. Phenolic and furanone glucosides were readily hydrolyzed by saliva and skin microflora, whereas monoterpenol glycosides were poorer substrates for the enzymes involved. The results of this comparative analysis of SMGs provide valuable information for elucidating the biological functions of SMGs and the future technological applications of these useful natural products.

Genome-wide characterization of NAC transcription factors in Camellia sinensis and the involvement of CsNAC28 in drought tolerance

The NAM, ATAF1/2, and CUC2 (NAC) transcription factors, which are members of a plant-specific gene family, play critical roles during the growth and development of plants and in their adaption to environmental stress. Few NAC transcription factors have been functionally characterized in tea plants (Camellia sinensis). Based on the analysis of the gene structure, motif pattern, and evolutionary relationship, we identified 104 NAC genes in C. sinensis. Among them, CsNAC28 is constitutively expressed in all organs, and most significantly, exhibiting remarkable responsiveness to abscisic acid (ABA) treatment and drought stress. ABA is a primary stress-related hormone. Recently, ABA-responsive element binding factor 2 (CsABF2) was identified in the ABA pathway of C. sinensis. However, the involvement of the CsABF2-mediated ABA pathway in regulating CsNACs was not known. Herein, a series of biochemical and genetic approaches supported the fact that CsNAC28 could potentially act as a transcription factor in the downstream of CsABF2. Furthermore, we investigated the function of CsNAC28 in the adapting of a plant to drought stress. The results showed that overexpression of CsNAC28 in Arabidopsis conferred hypersensitivity to ABA treatment and decreased the accumulation of reactive oxygen species (ROS), resulting in improved dehydration tolerance. Under conditions of drought, the expression levels of ABA pathway-related genes and drought stress‒inducible genes were greater in CsNAC28 overexpression lines than in the wild type. Our study's comprehensive characterization of NAC genes in C. sinensis could serve as a foundation for exploring the molecular mechanism of CsNAC-mediated drought responsiveness.

CsGDH2.1 negatively regulates theanine accumulation in late-spring tea plants (Camellia sinensis var. sinensis)

Theanine, a unique and the most abundant non-proteinogenic amino acid in tea plants, endows tea infusion with the umami taste and anti-stress effects. Its content in tea correlates highly with green tea quality. Theanine content in new shoots of tea plants is high in mid-spring and greatly decreases in late spring. However, how the decrease is regulated is largely unknown. In a genetic screening, we observed that a yeast mutant, glutamate dehydrolase 2 (gdh2), was hypersensitive to 40 mM theanine and accumulated more theanine. This result implied a role of CsGDH2s in theanine accumulation in tea plants. Therefore, we identified the two homologs of GDH2, CsGDH2.1 and CsGDH2.2, in tea plants. Yeast complementation assay showed that the expression of CsGDH2.1 in yeast gdh2 mutant rescued the theanine hypersensitivity and hyperaccumulation of this mutant. Subcellular localization and tissue-specific expression showed CsGDH2.1 localized in the mitochondria and highly expressed in young tissues. Importantly, CsGDH2.1 expression was low in early spring, and increased significantly in late spring, in the new shoots of tea plants. These results all support the idea that CsGDH2.1 regulates theanine accumulation in the new shoots. Moreover, the in vitro enzyme assay showed that CsGDH2.1 had glutamate catabolic activity, and knockdown of CsGDH2.1 expression increased glutamate and theanine accumulation in the new shoots of tea plants. These findings suggested that CsGDH2.1-mediated glutamate catabolism negatively regulates theanine accumulation in the new shoots in late spring, and provides a functional gene for improving late-spring green tea quality.

Transcriptome Sequence Analysis of the Defense Responses of Resistant and Susceptible Cucumber Strains to Podosphaera xanthii

Powdery mildew (PM) caused by Podosphaera xanthii poses a continuous threat to the performance and yield of the cucumber (Cucumis sativus L.). Control in the initial stages of infection is particularly important. Here, we studied the differential physiological and transcriptomic changes between PM-resistant strain B21-a-2-1-2 and PM-susceptible strain B21-a-2-2-2 at the early stage of P. xanthii attack. When challenged with P. xanthii, the tolerant line can postpone the formation of the pathogen primary germ. Comparative transcriptomic analysis suggested that DEGs related to the cell wall and to pathogen and hormone responses were similar enriched in both cucumber lines under P. xanthii infection. Notably, the number of DEGs triggered by P. xanthii in B21-a-2-1-2 was quintuple that in B21-a-2-2-2, revealing that the success of defense of resistant cucumber is due to rapidly mobilizing multiple responses. The unique responses detected were genes related to SA signaling, MAPK signaling, and Dof and WRKY transcription factors. Furthermore, 5 P. xanthii -inducible hub genes were identified, including GLPK, ILK1, EIN2, BCDHβ1, and RGGA, which are considered to be key candidate genes for disease control. This study combined multiple analytical approaches to capture potential molecular players and will provide key resources for developing cucumber cultivars resistant to pathogen stress.

Eugenol functions as a signal mediating cold and drought tolerance via UGT71A59-mediated glucosylation in tea plants

Cold and drought stress are the most critical stresses encountered by crops and occur simultaneously under field conditions. However, it is unclear whether volatiles contribute to both cold and drought tolerance, and if so, by what mechanisms they act. Here, we show that airborne eugenol can be taken up by the tea (Camellia sinensis) plant and metabolized into glycosides, thus enhancing cold and drought tolerance of tea plants. A uridine diphosphate (UDP)-glucosyltransferase, UGT71A59, was discovered, whose expression is strongly induced by multiple abiotic stresses. UGT71A59 specifically catalyzes glucosylation of eugenol glucoside in vitro and in vivo. Suppression of UGT71A59 expression in tea reduced the accumulation of eugenol glucoside, lowered reactive oxygen species (ROS) scavenging capacity, and ultimately impaired cold and drought stress tolerance. Exposure to airborne eugenol triggered a marked increase in UGT71A59 expression, eugenol glucoside accumulation, and cold tolerance by modulating ROS accumulation and CBF1 expression. It also promoted drought tolerance by altering abscisic acid homeostasis and stomatal closure. CBF1 and CBF3 play positive roles in eugenol-induced cold tolerance and CBF2 may be a negative regulator of eugenol-induced cold tolerance in tea plants. These results provide evidence that eugenol functions as a signal in cold and drought tolerance regulation and shed new light on the biological functions of volatiles in the response to multiple abiotic stresses in plants.

Accumulation of Salicylic Acid and Related Metabolites in Selaginella moellendorffii

Salicylic acid (SA) is a phytohormone that plays manifold roles in plant growth, defense, and other aspects of plant physiology. The concentration of free SA in plants is fine-tuned by a variety of structural modifications. SA is produced by all land plants, yet it is not known whether its metabolism is conserved in all lineages. Selaginella moellendorffii is a lycophyte and thus a representative of an ancient clade of vascular plants. Here, we evaluated the accumulation of SA and related metabolites in aerial parts of S. moellendorffii. We found that SA is primarily stored as the 2-O-β-glucoside. Hydroxylated derivatives of SA are also produced by S. moellendorffii and stored as β-glycosides. A candidate signal for SA aspartate was also detected. Phenylpropanoic acids also occur in S. moellendorffii tissue. Only o-coumaric acid is stored as the β-glycoside, while caffeic, p-coumaric, and ferulic acids accumulate as alkali-labile conjugates. An in silico search for enzymes involved in conjugation and catabolism of SA in the S. moellendorffii genome indicated that experimental characterization is necessary to clarify the physiological functions of the putative orthologs. This study sheds light on SA metabolism in an ancestral plant species and suggests directions towards elucidating the underlying mechanisms.

Structural basis of NLR activation and innate immune signalling in plants

Animals and plants have NLRs (nucleotide-binding leucine-rich repeat receptors) that recognize the presence of pathogens and initiate innate immune responses. In plants, there are three types of NLRs distinguished by their N-terminal domain: the CC (coiled-coil) domain NLRs, the TIR (Toll/interleukin-1 receptor) domain NLRs and the RPW8 (resistance to powdery mildew 8)-like coiled-coil domain NLRs. CC-NLRs (CNLs) and TIR-NLRs (TNLs) generally act as sensors of effectors secreted by pathogens, while RPW8-NLRs (RNLs) signal downstream of many sensor NLRs and are called helper NLRs. Recent studies have revealed three dimensional structures of a CNL (ZAR1) including its inactive, intermediate and active oligomeric state, as well as TNLs (RPP1 and ROQ1) in their active oligomeric states. Furthermore, accumulating evidence suggests that members of the family of lipase-like EDS1 (enhanced disease susceptibility 1) proteins, which are uniquely found in seed plants, play a key role in providing a link between sensor NLRs and helper NLRs during innate immune responses. Here, we summarize the implications of the plant NLR structures that provide insights into distinct mechanisms of action by the different sensor NLRs and discuss plant NLR-mediated innate immune signalling pathways involving the EDS1 family proteins and RNLs.